Electrophoretic light modulators utilize electric fields to control the movement of electrically-charged particles that are typically suspended in fluid. In a typical, electrophoretic light modulator an electric force provided by a battery is used to cause such particles to move toward or away from a viewing surface. The electrodes in such a modulator may be made optically transparent and arranged as a parallel plate capacitor with the electrophoretic particles filling the void between the plates. In such a configuration, particles of differing color may be alternatively moved to or away from the viewing surface so that the color of certain portions of the surface change based on the color of the affected particles.
Unlike the structurally-similar liquid crystal light modulators, electrophoretic modulators transport material within a fluid to the viewing surface. Polarization effects may be eliminated if the particles themselves have no inherent anisotropic or polarization properties. The speed of an electrophoretic light modulator can exceed video rates. It is also possible to construct an electrophoretic light modulator to be bi-stable in its modulation behavior. That is, the electrophoretic particles can be made to remain at one optical surface or the other even after the excitation field has been applied and then removed. This is accomplished by coating the inner surfaces of the modulation cell with material that induce an electrostatic attraction between the particles and the inner surfaces when the particles close to within a certain range. The particles will then “stick” to the inner surface of the modulator cell until a reverse field breaks these weak Van der Waals bonds and pulls the particles away and back into suspension. This can result in optical displays with very low electrical power consumption. Electrophorectic light modulators are now making their way into the mainstream of optical displays under the general names of electronic paper or e-paper.
Despite their benefits, many electrophoretic modulators suffer from (1) limited or no ability to modulate certain wavelengths of light with high contrast, and (2) limited or no ability to reflect light in a specular manner. For example, it may be desirable for a light modulator to have very high specular reflectance in one state, and very high absorbance in another state. The use of electrophoretic particles can make it easy to obtain high optical absorption, but difficult to obtain high specular reflectance due to the random, diffuse nature of light scattering from an ensemble of small particles.